In video compression encoders, control of a degree of quantisation is an important means of controlling the coding process in order to match the capacity demands of the picture behaviour with the available output bit rate. A key parameter used extensively in current coding methods for practical compression applications (e.g. MPEG-2 and MPEG-4/H.264) is the Quantisation Parameter (Qp). Since an increased Qp value results in lower bit rates for the digital video bit stream, for a low target bit rate the value of Qp is at the high end of its nominal range (1 to 31 in MPEG-2, and 0 to 51 in MPEG-4/H.264). Conversely, where bit rate is not severely constrained, a value nearer to the lower end is chosen.
The process linking the target bit rate and the coding process, in particular the value of Qp, is known as Rate Control. The value of Qp is variable within each picture down to the level of a macroblock, but in simple coding implementations is often constant over a whole picture (i.e. non-adaptive quantization) and set at the value suggested by Rate Control. The result of such a simple implementation is that all the macroblocks in a picture contain a similar amount of distortion noise after decoding, regardless of how the macroblocks content and location can cover up the distortion, i.e. the choice of quantization parameter is done regardless of the behaviour of the image and without any recognition of the visible effect of the resultant coding distortions.
Adaptive quantisation, on the other hand, is used to distribute bit allocations within a picture without interfering with the Rate Control algorithm. In general, the picture quality can be improved by reducing the amount of quantisation in picture areas of lower spatial activity where the human visual system (HVS) can detect distortion more easily, whereas higher spatial activities are less affected by coarser quantisation because the HVS is less sensitive in picture regions with such high activity levels. However, while some picture sequences benefit from aggressive adaptive quantisation (i.e. large deviations from the average quantisation measured over the whole picture), others are better with little or no adaptive quantisation. This implies that some degree of control is necessary to adapt the value of Qp in a beneficial manner.
Adaptive Quantization is not a new idea and is in common use, but the prior art methods of Adaptive Quantization have been complex and hence costly to implement in terms of delays introduced into the video encoding pathway, or physical cost/size of the hardware required. Accordingly, the present invention seeks to describe a simple and cost effective method for controlling the aggression of adaptive quantisation.